It's not even monetary reason. It's literally to help keep this game relevant. This game would DIE if no new weapons and frames are released. It's on life support.
Most of the people here being sarcastic regarding your post haven't even played this game for 1 year and are talking as if they are some experts. Of course they would love if DE released a new frame every 3 months. They have just started this game. They have never used the outdated frames loki, frost, ember, etc. They get the new octavia, gara, etc. and see nothing wrong with this.
OP, it's life support for this game.
Edit: Most of the older players aren't even playing this game. I think another open world (Deimos) destroyed their desire to even continue with this.